CLADDING TECHNOLOGY SHANXI CO., LTDDeformation Analysis and Control Measures for Tube Sheet Overlay Welding
Literature Overview
This 1991 publication in Welding Technology (焊接技术) by researchers from Nanjing Second Chemical Machinery Factory addresses a critical practical problem in the fabrication of heat exchangers and pressure vessels: the control of deformation during and after overlay welding of tube sheets. Tube sheets are flat, thick plates with hundreds of precisely drilled holes, and their dimensional accuracy is essential for proper assembly with tube bundles and channel covers.
Core Technical Content
Tube sheets in heat exchangers and pressure vessels serve as the structural and sealing interface between the shell side and the tube side. They are typically manufactured from carbon steel or low-alloy steel and then clad with stainless steel or nickel-based alloy on one or both faces to provide corrosion resistance. The overlay welding of tube sheets presents unique challenges:
| Challenge | Description | Impact |
|---|---|---|
| Geometric complexity | Flat plate with hundreds of drilled holes | Holes act as stress concentrators and crack initiation sites |
| Dimensional accuracy | Tight tolerances on hole position, diameter, and flatness | Deformation can render the tube sheet unusable |
| Thick section | Typical thickness 20–60 mm | High restraint, high residual stress |
| Multi-pass overlay | Multiple layers required for full cladding thickness | Cumulative thermal cycling and distortion |
| Post-overlay machining | Surface must be machined to final thickness | Excessive deformation wastes material and machining time |
Deformation Mechanism Analysis
The deformation of tube sheets during overlay welding arises from several mechanisms:
- Thermal expansion and contraction: The localized heating of the weld zone causes thermal expansion, while the surrounding cooler material constrains this expansion. Upon cooling, the weld zone contracts but is restrained by the surrounding material, resulting in residual stresses and permanent deformation.
- Asymmetric heat input: When overlay welding is performed on one face of the tube sheet, the heat input is asymmetric, causing the tube sheet to warp or bow in the direction of the overlay.
- Restraint effects: The drilled holes in the tube sheet act as geometric discontinuities that locally increase restraint, concentrating residual stresses and potentially initiating cracks.
- Phase transformation: In low-alloy steels, the cooling of the heat-affected zone can induce martensitic transformation, which involves a volume expansion that further contributes to residual stress and deformation.
Control Measures
The authors describe several control measures, which can be categorized as follows:
Pre-Welding Measures
| Measure | Description | Effectiveness |
|---|---|---|
| Preheating | Uniform preheat to 100–200 °C | Reduces thermal gradient and cooling rate |
| Backing plate | Welding on a rigid backing plate | Provides support and reduces bowing |
| Welding sequence | Optimized multi-pass sequence to balance heat input | Minimizes asymmetric distortion |
| Tack welding | Securing the tube sheet to a fixture | Constrains movement during welding |
In-Process Measures
| Measure | Description | Effectiveness |
|---|---|---|
| Low heat input | Reduced current, increased travel speed | Limits thermal distortion |
| Multi-pass strategy | Multiple thin passes instead of few thick passes | Distributes heat input more evenly |
| Back-step welding | Starting and stopping at different locations | Reduces local stress concentration |
| Alternating sides | Welding on both faces alternately (if applicable) | Balances thermal input |
Post-Welding Measures
| Measure | Description | Effectiveness |
|---|---|---|
| Stress relief | Post-weld heat treatment at 550–650 °C | Reduces residual stresses |
| Mechanical straightening | Indenting or pressing to correct bowing | Can correct small deformations |
| Thermal straightening | Localized heating of convex areas | Can correct moderate deformations |
| Machining allowance | Designing for additional material to be machined away | Accounts for expected deformation |
Quantitative Deformation Analysis
The deformation of a tube sheet during overlay welding can be estimated using simplified beam theory. For a tube sheet of thickness t, width w, and length l, with an overlay layer of thickness d on one face, the expected bowing can be approximated as:
- Bowing direction: Toward the overlay side (the heated side contracts more upon cooling, pulling the sheet toward it)
- Bowing magnitude: Proportional to the ratio d/t and the total heat input
- Hole distortion: Local deformation around each hole, typically causing ovality and dimensional change
In practice, the deformation is rarely uniform — it varies across the tube sheet surface, with maximum bowing typically occurring near the center and reduced deformation near the edges (where restraint is greater due to the boundary conditions).
Engineering Practice Case
A typical tube sheet for a heat exchanger might have the following specifications:
| Specification | Value |
|---|---|
| Material | 16MnR (base) + 304 stainless steel (overlay) |
| Thickness | 40 mm |
| Diameter | 800 mm |
| Number of holes | 500 (diameter 25 mm) |
| Overlay thickness | 6 mm (3 passes of 2 mm each) |
| Acceptable flatness | ±1.0 mm over 800 mm diameter |
| Acceptable hole position tolerance | ±0.5 mm |
The welding sequence for such a tube sheet would typically involve:
- Preheat the entire tube sheet to 150 °C.
- Weld the first pass in a spiral or zigzag pattern, starting from the center and working outward.
- Grind flush after each pass.
- Allow the tube sheet to cool to ambient temperature between passes (or maintain at a controlled interpass temperature of 200–250 °C).
- After all passes are complete, perform stress relief at 600 °C for 2 hours per 25 mm of thickness.
- Machine the overlay surface to final thickness and verify flatness and hole dimensions.
Study Reflection
The 1991 publication of this work reflects an era when tube sheet fabrication was primarily manual or semi-automated, and deformation control relied heavily on empirical knowledge and experienced welder judgment. The systematic analysis of deformation mechanisms described in this paper represents a maturation of the field — moving from trial-and-error to understanding-based process control.
The key insight for modern practice is that tube sheet deformation is not merely a problem to be solved after it occurs — it is a predictable phenomenon that can be minimized through careful process design. The welding sequence, heat input, and post-weld treatment are not independent decisions; they must be optimized as a system to achieve the required dimensional accuracy.
In contemporary practice, finite element analysis (FEA) can be used to predict tube sheet deformation during overlay welding, allowing for virtual optimization of the welding sequence before any material is committed. However, the fundamental principles described in this 1991 paper — asymmetric heat input causes bowing, holes increase local restraint, and post-weld stress relief reduces residual stress — remain valid and are the basis for both empirical and computational approaches to deformation control.